- 



ISO 



i. a. in. 



Si0 2 ... 35-74 ... 37-09 ... 40-79 



Ti0 2 ... ... tr. 



A1 2 3 ... 12-23 ... 19-90 .. 10-41 



Fe a O a ... 4-68 ... 15-54 ... 3-52 



FeO ... 13-84 ... 2-02 ... fi-31) 



MnO ... 0-98 ... tr. 



CaO ... tr. ... tr. ... 8.48 



MgO ... 22-13 ... 15-90 ... 23-34 



K 8 ... tr. ... j \ ... 0-71 



Na a O ... 0-25 ... I J ... 1-71 



P 2 5 ... 0-18 ... -21 



H a O 10-01 8-65 4-04 



100-04 99-31 99-39 



Mr. PHILLIPS states that the specimen analysed by Mr. COLLINS was 

 harder and darker than the one he had analysed, and contained fewer of the 

 light coloui'ed pseudomorphs. It will be observed that Mr. COLLINS' analysis 

 shows less magnesia, a fact which agrees with the view that these 

 pseudomorphs are after olivine. Under the microscope the rock is seen to 

 consist mainly of a confused mass of fibres and scales ; some patches of this 

 give vivid aggregate polarisation, whereas the intervening portions are almost 

 isotropic. The only definitely recognisable minerals in the slide examined by 

 the writer are a brown mica and magnetite. It seems probable from the 

 amount of alumina in the rock that we have here a mixture of serpentine and 

 pseudophite, the serpentinous alteration product after felspar. The original 

 rock appears to have been a picrite, as may be seen by comparing the analysis 

 of Mr. PHILLIPS with that of the picrite from Giimbelberg. It is instructive 

 to notice, however, that the alteration of the Duporth picrite has given rise to 

 results the exact opposite of those described by TsCHBAMAK. In the picrites 

 of Moravia and Silesia the effect of the alteration has been to remove magnesia 

 and to replace it by lime. Here the lime has been almost entirely carried 

 away, while magnesia has remained or possibly even been introduced into the 

 rock. (1) It is also interesting to notice that the Duporth rock lends no 

 support to the view that a true serpentine may be produced by the alteration 

 of a felspathic rock. The original alumina has not been carried away. 



(I) The altered picrite from SShle yielded Si0 2 42-85, A1 2 3 10-42, Fe 2 O 8 6-27, FeO 6-86, 

 CaO 11-84, MgO 9-01, K a O 1-61, Na 2 O 1-65, H 2 O 2'70, CO Z 5-88. Total 09-09. Die 

 Porphyrgesteine Osterreichs, p. 252. 



